1. Field of the Invention
The present invention relates to apparatus and methods for selecting laser wavelengths by the use of resonantly moving intracavity elements. A primary objective is to obtain two selected wavelengths with high repetitive accuracy and maximum power output at high repetition rates.
Very high speed wavelength agility is achieved by the use of a resonant intracavity reflective element. With a resonant system, the cyclic frequency can be as short as one millisecond, so two stable selected optimum wavelength pulses can be fired at 500 microsecond intervals, an ideal timing for DIAL LIDAR operation. The features of the invention are applicable to both gas lasers and to some solid state nonlinear materials, used in solid state lasers and optical parametric oscillators (OPO).
2. Description of the Prior Art
Tunable lasers typically include a diffraction grating in the laser gain cavity to achieve wavelength tuning. In some cases an optical parametric oscillator (OPO) material (usually a crystal with highly non-linear optical properties) such as Beta Barium Borate is used. Tunable lasers may be tuned by precisely adjusting the angle of incidence of a laser cavity beam against the diffraction grating (or OPO crystal) in order to control or change the wavelength of light reflected or refracted and thereby tune the frequency of the laser output beam. Grating reflection angles have conventionally been adjusted by grating alignment micrometer screws. Manual micrometer adjustment is slow and severely restricts beam steering agility, and even automated micrometer adjustment by stepper motors is hindered by the angular momentum of the diffraction grating, screw friction, and other forces.
Laser radar (LIDAR) systems can be used to transmit different wavelengths of light into airborne suspensions (such as smog) which have differing reflectivities to the different wavelengths. Each wavelength's reflected light intensity is then measured for remote spectrographic analysis of suspension samples. In remote spectroscopy LIDAR applications it is advantageous to maximize the stability and repeatability of each laser pulse output at each different wavelength. On the other hand, it is very advantageous to minimize intervals between transmitting pulses in order to reduce measurement interference by relative motion between the LIDAR unit, the intervening atmosphere and the suspension sample. Maximum accuracy is achieved by successively transmitting the different wavelengths at the laser's maximum cyclic firing rate, which may vary from ten to hundreds or more of shots per second.
Referring to FIG. 1, laser 10 wavelength tuning rates have conventionally been somewhat increased by using continuously rotating polygonal mirrors 12 which sweep laser cavity beams 14 at different angles across one or more stationary diffraction gratings 16 to successively tune the laser output beam 18 to different predetermined wavelengths. However, the stable tuning capability of a continuously rotating polygonal mirror system is limited because the mirror facets 20 are never stationary. Triggering of laser pulses must be synchronized with the positions of facets 20 to initiate pulses when a facet is passing through the desired angular position corresponding to reflecting the wavelength desired for each pulse. A conventional polygonal mirror system for laser wavelength tuning uses a master clock 22, amplifier 24 and motor 26 to drive a massive polygonal mirror 12 at a constant speed. Clock sensor 28 uses rotation timing pickoff points which, with corresponding signals from desired position information memory 30, are input to delay circuit 32 to control a pulse trigger circuit 34 which triggers the laser pulses.
Referring to FIG. 2, the laser output pulse timing 36 is typically derived from the frequencies of the facet clock signals 38 and of the master clock signals 40 driving the polygonal mirror 12 at a nominally constant rate. Because polygonal mirror facets 20 continue moving as laser 10 is pulsed, continuously varying or smeared wavelengths are reflected from diffraction grating 16 back into the laser gain cell. These varying reflected wavelengths during pulses impose limitations on the laser's peak power and discrimination capability. Continuous rotation of polygonal mirrors degrades laser output pulse uniformity and mode stability, which also affects beam pointing. These effects increase proportionately with polygonal mirror rotation speeds.
Conventional optical element positioning systems are unable to tune lasers repeatably at high rates to high pulse power. High performance for maximum power optical beam tuning requires optical elements in the cavity to be stationary at the critical instant of laser pulse firing and then to jump quickly and precisely to the next position in a sequence which can be dynamically altered in accordance with changes in system operating conditions, which is not feasible with conventional optical element positioning systems.